1. Field of the Invention
This invention relates to thermally treating mechanically dewatered sludge from municipal wastewater treatment plants, pulp and paper mills, oil refineries, food processing plants, or pharmaceutical manufacturing plants for removing water from fibrous suspensions. More specifically, this invention relates to a sludge drying system for producing granular particles or pellets to be used as fertilizer, and to a classifying system for pellet separation.
2. Description of the Prior Art
Typically, sludge from a municipal wastewater treatment plant or paper mill is first mechanically dewatered. The mechanically dewatered sludge is then fed to a sludge drying plant which utilizes heated air in a rotating drying drum. Dried sludge is pelletized and can be either sold as fertilizer or disposed of as permitted by restricting regulations.
Many processes have been proposed for thermally treating mechanically dewatered sewage sludge in a drying system. One such process is described in VSA - Dokumentation, "Klarschlammbehandeln" by U. Keller (1981) in which large or oversized dried sludge pellets are crushed and mixed with the undersized pellets, and then recycled for mixture with incoming dewatered sludge. This process has the drawback that large quantities of oversized pellets are crushed into smaller size particles or fines which are then recycled. This process requires high energy input for the crusher and also for the transporting devices. Also this large quantity of crushed sludge particles has to be heated in the dryer which also results in an increase energy consumption for the entire system.
This invention addresses these problems in the art as well as other problems in the art which will be apparent to those skilled in the art once given this disclosure.